Land, which now covers about a third of the planet, remains
devoid of life during the Cambrian period. Remnants of Rodinia, the dominant
landmass during the late Proterozoic era, drift further apart. Where moving
tectonic plates collide, mountain ranges form, especially on Gondwana, the largest
of the fragments.

Atmospheric and marine oxygen levels reach new heights in the
generally warm and stable climate. These environmental factors may help trigger
an "explosion" in animal diversity. In the seas, some three dozen animal phyla
first appear, each with a distinctive body plan. While several of these groups
disappear over time -- beginning with a series of extinction episodes near the end
of the Cambrian -- most persist (though greatly modified) to the present day.

543 mya:Hard-shelled animals

Beginning in the early Cambrian, many animals evolve hard external skeletons of all shapes and sizes that shield and support their bodies. Shells, tubes, and spines, made primarily of calcium phosphate or calcium carbonate, comprise a major part of the fossil record.

540 mya:Oldest arthropod fossils

Arthropods, which include the insects and crustaceans, will become the most diverse phylum both on land and in the oceans. They are known for their segmented bodies, jointed legs, well-defined head area, and hard outer coating, or cuticle. While the oldest definitive arthropod fossils, which were left in abundance by communities of bottom-dwelling sea creatures called trilobites, date to the early Cambrian, trace fossils suggest they may have appeared even earlier, during the Vendian period.

535 mya:Chordates

Members of the phylum Chordata, which include humans and all other vertebrates, have a stiff yet flexible rod that runs down the middle of the back, called a notochord. Made of cartilage-like material, not bone, the notochord provides support for the nerve cord. Other features characteristic of the chordate body plan are a tail and gill-like openings between the throat and the neck. In many chordates like the vertebrates, some of these features are apparent only in the early stages of embryonic development.

Did you know?

The oldest-known fossil chordate comes from the Chengjiang
find in southern China. Like Pikaia, a two-inch creature from another
find in Canada's Burgess Shale rock formation, Cathaymyrus is a small,
wormlike marine creature. It is thought to be a fossil relative of modern
lancelets, which are small, tapered filter feeders that live buried in the
sand in shallow waters. Either Pikaia or Cathaymyrus -- or both
-- may be the ancestors of all vertebrates.

530 mya:The Cambrian explosion

The basic body plans of the major animal phyla are established
over a relatively short period of roughly 10 million years. All the major animal
phyla that exist today -- about three dozen -- evolve from these Cambrian faunas.

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The Cambrian explosion (530-520 mya)

While scientists now know that animal life existed prior to
the Cambrian explosion, the diversity of life that evolves during its 10 million
years remains significant. While the soft-bodied Ediacaran animals had no protective
coverings, many Cambrian animals evolved skeletons, such as shells or other brittle
coatings. Among the more familiar groups to appear include sponges,
brachiopods (lamp shells), spiny-skinned echinoderms,
early gastropods (snails), cone-shelled cephalopods, and
primitive arthropods called trilobites. Several other creatures that are
unrelated to any currently living form also appear, but they die off after a short
time.

A combination of environmental factors probably contributes to
this evolutionary burst. Oxygen, which is plentiful in both the atmosphere and in
the oceans, allows physically larger animals to evolve. Warm, free-flowing ocean
currents probably carry these animals to new marine niches, where they adapt to new
settings and evolve different characteristics.

Environmental factors alone, however, cannot explain why major
animal phyla have not evolved in the 500 million years or so since the
Cambrian explosion. Studies comparing fossilized embryos and a wide range of
contemporary specimens suggest that homeobox genes -- genes that control whether
certain cells specialize to form muscles, nerves, or glands, for example -- are
remarkably similar in all species. The mutations that give rise to these control
genes may be advantageous only in the earliest, simplest animals.

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Body plans

Members of a phylum share basic structural similarities,
or "body plans." Representatives of the phylum Chordata, for instance, have nerve
cords that run along their backs. But all chordates don't look the same. Certain
individuals use fins and a tail to get around, while others use limbs.

At least 10 million animal species may exist today, yet there
are only about 30 different phyla. How can we have such species variety within a
limited set of body plans? Members of a phylum inherit the same basic genetic tool
kit. Over time, some evolve with modified features and became new species. It might
help to think of a phylum as a prototype car design and individual species as
different models.

Fossils of the Burgess Shale

The Cambrian explosion is documented in an extraordinary fossil
find. The Burgess Shale rock formation, discovered in 1909 in western Canada,
contained examples of the variety of life that existed in the Cambrian period.
These precursors of all life to come were preserved -- hard and soft parts alike --
in the fine mud that buried them in an underwater avalanche.

530 mya:Marine reefs

Marine reefs are important ecosystems that support a wide variety of
organisms. They form in shallow, tropical waters and are highly sensitive to changes in
ocean conditions, like fluctuations in temperature and sea level. The archaeocyathids,
spongelike animals that build the first reefs, go extinct only 10-15 million years after
they first appear. Among the major reef-building groups that succeed the archaeocyathids
include stromatoporoids and, much later, corals.

500 mya:Gondwana forms

A remnant of the Rodinia supercontinent, Gondwana is the southern
hemisphere's primary landmass throughout the Paleozoic era. The present-day continents
Africa, Antarctica, Australia, and South America, as well as the subcontinent India,
were originally parts of Gondwana.

Cambrian extinction

Date:

520 mya

Intensity:

3

Affected:

Reef-builders and other shallow-water
organisms go extinct

Hypotheses:

Sea-level changes, oxygen depletion

Summary:

While the Cambrian period is witness
to the evolution of several major animal groups, two extinction events -- the first
coming about 520 mya -- each knock out 40-50 percent of marine genera. Tropical,
shallow-water bottom dwellers are hit first, most likely due to an upwelling of cold,
oxygen-poor waters from the deep sea. Then, the early reef builders perish when the
sea level drops and the continental shelves -- their primary habitats -- are exposed.
Successor reef-building animals, which include stromatoporoids and
colonies of tiny "moss animals" called bryozoans, do not appear until the
Ordovician period.